The present invention relates to a substantially reflectionless transducer structure for surface acoustic wave devices and in particular to a single level surface acoustic wave transducer having quarter wave length electrodes which substantially eliminates electrode reflections and which can be easily made unidirectional.
Surface acoustic wave devices known as SAW devices have many uses in the UHF and VHF frequency ranges. SAW devices have been especially useful as impedance elements, resonators, and band pass filters in these frequency ranges. Typical SAW devices have a substrate with at least a surface layer of piezo-electric material and surface acoustic wave transducers in interdigitated form disposed on the piezo-electric surface. The transducers convert an electrical signal to surface acoustic waves propagating on the piezoelectric surface. Several problems are associated with prior art .lambda./4 surface acoustic wave transducers. One of the problems occurs because the transducer electrodes cause internal reflections which distort the transducer output and the shape of the input conductance. Another problem occurs when the transducer is used in filter applications. Triple transit distortion is caused by regeneration reflections in the transducers.
The first problem, distortion caused by internal reflections, is solved in the prior art by the use of structures having 3 or 4 electrodes per wavelength to cancel the internal reflections and provide a symmetrical input conductance wave shape. However, the increased number of electrodes per wavelength limits the frequency of operation of the structure due to photolithographic constraints.
In order to eliminate triple transit distortion, three-phase and single-phase devices are used to form unidirectional transducers. Again, the size of the electrodes becomes a limiting factor in the construction of the device and thereby limits the frequency of operation of the device.
The present invention proposes a simple single level interdigitated SAW transducer having .lambda./4 electrodes in which reflections are canceled and therefore allow a structure to be built at twice the frequency of those presently available. The device has an undistorted output and a symmetric input conductance, can be placed on standard crystal cuts and can be made into single-phase unidirectional transducer as a two level structure in a simple manner. Further the single-phase transducer can be made with a flat input susceptance and symmetric input conductance for use as impedance elements.
Thus all electrodes in the transducer are .lambda./4 and the gaps between electrodes in the transducer are .lambda./4 or .lambda./2. The electrodes of the transducer are on a fixed grid for relatively easy construction.
Electrode reflections in the transducer are completely canceled by destructive interference on a local basis. Reflections of each electrode in the transducer are canceled by the reflections of a neighboring electrode a distance of .lambda./2 away or a multiple thereof. The transducer utilizes a plurality of groups of interdigitated .lambda./4 electrodes extending from opposed transducer pads to form an elongated transducer. Each group of the interdigitated electrodes has a spacing within the group of .lambda./4 and the spaces between the adjacent groups of the interdigitated electrodes are .lambda./2. With this construction, adjacent groups cancel reflections from each other. The groups can consist of any number of electrodes, n, where n is equal or greater than two. Thus electrode groups may consist of two, three, four or more electrodes per group.
In addition the structures can be made unidirectional by mass loading of alternate groups. Further, if the mass loading of the alternate groups is reversed, the sense of the unidirectionality also changes.
The frequency response of the two electrodes per group device includes group type responses which are 40% above and below the pass band. While this is a disadvantage, the responses are sufficiently far removed from the pass band for most filtering applications. Also, as the number of electrodes per group increases, the group type responses move closer to the pass band. This allows coupled resonators and filters to be constructed with one transducer having a first number of electrodes n per grouping and a second transducer to be constructed of an electrode grouping having a different number of electrodes thereby allowing good communication between the transducers in the pass band but poor communication between the transducers out of the pass band.
Further, by adding an additional interdigitated electrode in alternate ones of said groups of n electrodes a weakly unidirectional transducer is obtained
Thus it is an object of the present invention to provide a single level surface acoustic wave transducer which eliminates electrode reflections.
It is also an object of the present invention to provide a surface acoustic wave device having .lambda./4 electrodes which eliminate electrode reflections and can easily be made unidirectional with a flat input susceptance and a symmetrical input conductance.
It is yet another object of the present invention to provide a surface acoustic wave device having .lambda./4 electrodes on a standard crystal cut which eliminates electrode reflections, has a symmetric input conductance and an undistorted output.
It is still another objection of the present invention to provide a surface acoustic wave transducer having a plurality of groups of n interdigitated .lambda./4 electrodes extending from opposed conductive pads with each group of the interdigitated electrodes having a spacing within the group of .lambda./4 and the spaces between adjacent groups having a spacing of .lambda./2 for maximum coupling between the electrodes and the substrate.
It is still another object of the present invention to provide either an impedance element or a single port resonator with a transducer formed of .lambda./4 electrodes.